There are replacement parts available, where do the parts that Google has used to send out reman phones come from? Just because one of their authorized places does not have the part, does not mean they are not available. is there a good supply? But you cannot remain an item without parts and for sure they do not return a phone to an assembly line for repair. And as parts are shipped to the manufacturing line, some will be shipped to a parts warehouse somewhere in the world.
If you do a search you might find the part on the market (I just found one). From looking at the teardown down over on JerryRig; this screen is about as easy as any I have seen and have done (older phones for friends) and I have a bad habit of taking apart older phones before hitting the dumpster.
As for replacement parts, generally, the company has replacement parts that are supplied. When a phone is returned it is sent a reman center, either owned by the manufacture or by an approved second company (in this case google would use an authorized repair facility), they test, replace parts and send to a warehouse and sent out as a replacement. In other words, those non-NEW, replacement phones are rebuilt.
Also, for a large company, when a phone is returned, even for a protection warranty for a screen, the warehouse it is returned to sends it to the repair center and ships a rebuilt to the customer they do not repair in the warehouse. And if they did, that could add days or weeks to even a simple repair.
So to sum it up; if google has the parts for remaning a returned phone, there are parts.